1. Field of the Invention
The invention relates to an ink-jet apparatus and a driving method thereof.
2. Description of Related Art
Non-impact printers are currently expanding their markets, taking the place of impact printers already on the market. Of the various types of non-impact printers, an ink-jet printer is based on the simplest principle and can easily implement multiple gradations and color printing. Among the ink-jet printers, a drop-on-demand ink-jet printer, which ejects only ink droplets to be used in printing, is rapidly coming into wide use because of its superior ejecting efficiency and inexpensive running cost.
A Kyser ink-jet printer disclosed in U.S. Pat. No. 3,946,398 and a thermal-jet printer disclosed in Japanese unexamined Patent Publication No. 55-27282 are known as representative drop-on-demand type ink-jet printers. It is difficult to reduce the size of the former printer, and the ink used in the latter printer is required to have heat resistance because the ink is subjected to a high temperature. For these reasons, each of the printers has its own very difficult problem.
A shear mode jet printer as disclosed in U.S. Pat. No. 4,879,568 that utilizes piezoelectric ceramics is proposed as a new method for solving the problems of the prior art.
As shown in FIGS. 1 and 2, a shear mode ink-jet apparatus 600 comprises a bottom wall 601, a top wall 602, and shear mode actuator walls 603. Each actuator wall 603 is composed of a lower wall 607 which is bonded to the bottom wall 601 and is polarized in the direction designated by an arrow 611, and an upper wall 605 which is bonded to a top wall 602 and is polarized in the direction designated by an arrow 609. Each pair of actuator walls 603 constitutes an ink flow passage 613 between the upper and lower walls. A space 615 which is narrower than the ink flow passage 613 is formed between each adjacent pair of actuator walls 603.
A nozzle plate 617 having a nozzle 618 formed therein is fixedly provided at one longitudinal end of each ink flow passage 613. An electrode 619 is provided on one side of the actuator wall 603 in the form of a metallized layer, and another electrode 621 is provided on the other side of the actuator wall 603, also in the form of a metallized layer. Specifically, the actuator wall 603 of the ink flow passage 613 is provided with the electrode 619, and the actuator wall 603 of the space 615 is provided with the electrode 621. The surface of the electrode 619 is coated with an insulating layer 630 so as to isolate the electrode surface from ink. The electrode 621 is provided so as to face the space 615 and is grounded to an earth ground 623. The electrode 619 provided in the ink flow passage 613 is connected to a control circuit 625 which outputs an actuator drive signal.
The manufacture of the ink-jet apparatus 600 will now be described. A piezoelectric ceramics layer polarized in the direction designated by the arrow 611 is bonded to the bottom wall 601, and a piezoelectric ceramic layer polarized in the direction designated by the arrow 609 is bonded to the top wall 602. The thickness of the respective ceramics layers is substantially equal to the height of the lower wall 607 and the upper wall 605. Parallel notches are then cut in the piezoelectric ceramics layers by rotation of a diamond cutting disk, whereby the lower wall 607 and the upper wall 605 are formed. The electrode 619 is deposited on the side surface of the lower wall 607 by vapor deposition, and the electrode 619 is further coated with the insulating layer 630. Similarly, the electrode 621 is formed on the side surface of the upper wall 605.
The peaks between the notches of the upper wall 605 and the lower wall 607 are bonded together, so that the ink flow passage 613 and the space 615 are formed. The nozzle plate 617 having the nozzle 618 formed therein is bonded to one longitudinal end of each of the ink flow passage 613 and the space 615 in such a way that the nozzle 618 corresponds to the ink flow passage 613. The other longitudinal ends of the ink flow passage 613 and the space 615 are connected to the control circuit 625 and the earth ground 623, respectively.
As a result of the application of a voltage from the control circuit 625 to the electrode 619 of each ink flow passage 613, the actuator wall 603 causes piezoelectric thickness deformation in such a direction that the volume of the ink flow passage 613 increases. For example, if a voltage E (V) is applied to an electrode 619C of an ink flow passage 613C, as shown in FIG. 3, an electric field develops in respective actuator walls 603E and 603F in the directions designated by arrows 627 and 629, as a result of which the actuator walls 603E and 603F cause piezoelectric thickness deformation in such a direction that the volume of the ink flow passage 613C increases. At this time, the pressure within the ink flow passage 613C including the vicinity of a nozzle 618C drops. The decreased pressure is held for time T during which a pressure wave uni-directionally and longitudinally travels along the inside of the ink flow passage 613. During this period, ink is fed from a manifold (not shown) to the ink flow passage 613.
The time T is necessary for the pressure wave to travel along the ink flow passage 613 in a longitudinal direction thereof. The uni-directional propagation time T is determined by the length L of the ink flow passage 613 and the speed of sound "a" in the ink within the ink flow passage 613. Specifically, the uni-directional propagation time T is defined as T=L/a.
According to the theory of propagation of pressure waves, the pressure within the ink flow passage 613 is reversed immediately after the time T has elapsed since the application of the pressure, whereupon the pressure changes so as to become positive. The voltage applied to an electrode 619C of the ink flow passage 613C is reset to 0 (V) in accordance with the inversion of the pressure from negative to positive.
As a result, the actuator walls 603E and 603F return to their original states as shown in FIG. 1, and the ink is pressurized. At this time, the pressure that became positive, and the pressure developed as a result of the actuator walls 603E and 603F returning to their original states, are added to each other, and a relatively high pressure develops in the vicinity of the nozzle 618C of the ink flow passage 613C. Eventually, the ink is ejected from the nozzle 618C.
According to the ink-jet apparatus having the above-described construction and a driving method thereof, it is possible to provide the ink within the ink flow passage 613C with such a relatively high pressure as previously mentioned at the moment the ink droplet is squirted from the nozzle 618C.
With reference to FIGS. 4A to 4G, a meniscus of the ink formed in the nozzle 618 will be described. As shown in the drawings, the meniscus of the ink changes with time (t=0-6T).
In FIG. 4B, the meniscus 24 of the ink recedes into the inside of the nozzle 618C. Some of the previously mentioned high pressure developed to eject the ink is wasted in pushing the meniscus 24 back to the nozzle exit, and hence the high pressure that contributes to the ejecting of the ink droplets is reduced. For this reason, if it is necessary to eject a large amount of ink, the required volume of the ink will not be obtained, thereby resulting in poor print quality.